Introduce `system.client_routes`, a system table that sets the target
address and ports for each `host_id`, for one or more connections
(e.g., Private Link) represented by `connection_id`. Cloud will write
the table via REST, and drivers will read it via CQL to override
values obtained from `system.local` and `system.peers`.
The table is Raft-managed to provide consistent replication across
nodes.
Schema overview: each row is identified by `(connection_id, host_id)`
and describes where clients should connect: `address` and one or more of
`port`, `tls_port`, `alternator_port`, `alternator_https_port`.
`host_id` is a UUID (just as in ScyllaDB) but `connection_id` can be
any string to accept formats of all cloud providers. `address` is
also a regular string because it can represent either an IP address or
a domain. Ports are optional in the sense that at least one of
the four must be provided.
Ref: scylladb/scylla-enterprise#5699
8dde70d04c